Philadelphia climate

Philadelphia Frost Dates and Growing Season

NOAA 1991-2020 normals for Philadelphia 1 Wsw, 1.3 km from Philadelphia, elevation 413 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Philadelphia 1 Wsw. A median date has an even chance of a later freeze in spring, or an earlier one in fall.

The last spring freeze typically comes around March 20 and the first fall freeze around November 9, a growing season of about 235 days.

Median at 36°F
last April 3, first October 30
Median at 32°F
last March 20, first November 9
Median at 28°F
last March 4, first November 21

Last freeze in spring

The chance of a later freeze than the date shown. At 10 percent, one spring in ten still freezes after the date; planting after the 10 percent date for 32°F is the cautious choice.

Threshold10%20%30%40%50%60%70%80%90%
36°FApr 21Apr 14Apr 10Apr 6Apr 3Mar 31Mar 27Mar 21Mar 15
32°FApr 9Apr 3Mar 30Mar 25Mar 20Mar 16Mar 11Mar 6Feb 28
28°FMar 25Mar 18Mar 13Mar 8Mar 4Feb 28Feb 24Feb 19Feb 11
24°FMar 12Mar 5Feb 28Feb 23Feb 16Feb 10Feb 3Jan 30Jan 20
20°FMar 2Feb 22Feb 11Feb 3Jan 30Jan 26Jan 20Jan 11Jan 1
16°FFeb 17Feb 5Jan 31Jan 27Jan 22Jan 17Jan 12Jan 6Dec 30

First freeze in fall

The chance of an earlier freeze than the date shown. At 10 percent, one fall in ten freezes before the date.

Threshold10%20%30%40%50%60%70%80%90%
36°FOct 17Oct 21Oct 24Oct 27Oct 30Nov 1Nov 4Nov 7Nov 14
32°FOct 26Oct 31Nov 2Nov 5Nov 9Nov 12Nov 16Nov 21Nov 28
28°FNov 3Nov 9Nov 13Nov 17Nov 21Nov 26Dec 1Dec 5Dec 14
24°FNov 13Nov 22Nov 29Dec 3Dec 7Dec 15Dec 24Jan 1Jan 10
20°FDec 1Dec 9Dec 19Dec 27Jan 1Jan 5Jan 10Jan 16Jan 30
16°FDec 19Dec 27Jan 1Jan 6Jan 9Jan 14Jan 19Jan 26Feb 5

Growing season length

Days between the last spring and first fall freeze at each threshold, with the chance of a season at least that long.

Threshold10%20%30%40%50%60%70%80%90%
36°F235225220215210206202196190
32°F261253246239235229223217209
28°F292281274267263257251244236
24°F332318310303296289282273262
20°F365352343336331323316306294
16°F365365365365360351342334323

How often it gets that cold

The share of years with at least one night at or below each temperature.

36°F or colder
100% of years
32°F or colder
100% of years
28°F or colder
100% of years
24°F or colder
99% of years
20°F or colder
85% of years
16°F or colder
54% of years

Degree days

A degree day is one degree of the daily mean temperature beyond a base, for one day. Heating degree days count cold (the fuel a house burns), cooling degree days heat, growing degree days the warmth a crop needs; each crop has its own base.

Heating degree days, base 65°F
2,444
Cooling degree days, base 65°F
2,174
Growing degree days, base 40°F
8,980
Growing degree days, base 45°F
7,325
Growing degree days, base 50°F
5,807
Growing degree days, base 55°F
4,437
Growing degree days, base 60°F
3,223

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January606372
February4456100
March28725249
April11677416
May20226671
June0404853
July0498963
August0485950
September5323768
October102105473
November32716195
December537797

What this means

NOAA computes these from the daily lows at the station over 1991-2020, fitting the dates of each year's last and first freeze to a distribution, so a date can be stated at any level of risk. 36°F is a frost risk for tender plants on a clear, calm night, since the ground cools below the air above it; 32°F is a freeze; 28°F is a hard freeze that kills most annuals.

The station is 1.3 km from Philadelphia. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

More

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00226894 (Philadelphia 1 Wsw). Public domain.